Mountain bike shoe soles to be used with clip-in pedals typically include a recess in the forward center part (under the ball of the foot) that has room to fit a two-hole pedal cleat (usually called an SPD compatible cleat).
On the inner side of the sole, a threaded fastener for the cleat is provided, which includes two or more female threads; such fastener can usually slide fore and aft and slightly rotate so that, when the SPD compatible cleat is connected to the fastener by means of two (or more) screws, the mounted position of the cleat itself can be adjusted to user preference.
Since the shoe sole is intended for use on mountain bikes, the sole has a tread for walking traction and for properly interfacing with the pedal.
The bicycle pedal correspondingly includes an engagement mechanism that engages with the cleat, after the latter has been rigidly mounted to the sole of the cycling shoe.
The stability of the pedal/shoe interface depends on the pedal mechanism being engaged with the cleat, and simultaneously the pedal body contacting the tread of the sole that is adjacent to the cleat mount area.
As the tread wears, the pedal/shoe interface becomes more and more sloppy, and less and less desirable by the rider.
Many mountain bikers combine riding their bike with “hike a bike”, which is carrying their bike over certain parts of a trail, and this usually causes significant tread wear.
With enough tread wear, the pedal/shoe interface becomes overly sloppy, and the shoe must be discarded even if it is in excellent conditions other than the tread that is directly adjacent to where the cleat is mounted.
Currently, there is no clear way for riders to know when their shoe tread has worn beyond its useful life.
Some prior art shoes have replaceable tread pieces, but they also include lots of fasteners holding down the many different tread pieces, so that the solution results in inferior tread, and too many screw heads are positioned where tread rubber would be preferred.
Generally, being clipped into a pedal enables the rider to both push and pull during pedal strokes, allowing the most possible power input and efficiency. However, sometimes riders chose not to be clipped into their pedal because of safety concern, or inability to clip into their pedal when first starting out from a standstill, or various other situations.
For example, many mountain bike riders occasionally want to traverse a difficult part of the trail, and the rider may prefer to be able to immediately remove their foot from their pedal without having to first twist out to disengage.
In other cases, riders place their foot on the center area of their shoe, so that they can apply weight to the pedal without risk of clipping into the cleat.
However, since clip-in pedals have a mechanism that protrudes above the pedal body, standing on the pedal means, for the rider, standing on the pedal mechanism and this is awkward, unstable and it can cause the shoe to inadvertently slip off the pedal.
When first starting out from a standstill—and with one shoe already clipped into one pedal—and especially when starting out up or downhill, it is often difficult, for the rider, to immediately clip into the other pedal; then the rider typically puts the middle area of his shoe onto the pedal, until an opportunity comes to clip into that pedal.
As mentioned above, this can be awkward, unstable, and cause the shoe to slip off the pedal.